The word artificial refers to the electromagnetic properties of homogeneous surfaces and materials that are not naturally observed in nature. The macroscopic electromagnetic properties of these homogeneous materials are determined by their microscopic structures. Therefore it is convenient to call these surfaces and materials also as metasurfaces and metamaterials, which are the common names in the literature for the surfaces and materials.
An artificial impedance surface can be created by metal patterning on a dielectric surface above a ground plane. By varying the local size and spacing of the metal patterning, specific reactive impedance values can be obtained. To scatter a given excitation from the artificial impedance surface into a desired far field pattern, one can use a holographic technique to determine the required space-dependent impedance function, and in turn the local metal patterning necessary to create the desired impedance function.
In the area of holographic antennas, holograms are built from cylindrical surface waves generated by point-sources, leading to low efficiency. In addition, reflections from the edges of the surface do not radiate in the prescribed direction. The described approach in U.S. Pat. No. 7,929,147 B1 revises the prescribed surface impedance distribution in U.S. Pat. Nos. 7,911,407 and 7,830,310 B1 to account for edge reflections, but achieves only moderate improvements in efficiency since the hologram is still essentially built from cylindrical surface waves as the source, and modifying the hologram to account for the edge reflections necessarily reduces the efficiency for radiating the initial cylindrical wave front. The design in US 2013/0285871 A1 achieves the goal of generating a 2D surface plane wave from a point-source, however it captures only a small fraction of the source energy and it adds significantly to the size of the antenna. It uses a long tapered transmission line as a feed, but its length can easily be multiple times that of the actual antenna, limiting its practical usefulness.
The prior art techniques suffer from poor efficiency in the transformation of source energy to radiated energy, require relatively larger feed and/or radiating surface and suffer from beam distortions due to edge reflections from the radiating surface. In addition, the prior art techniques suffer from poor control in focusing the radiated energy in the prescribed direction of radiation.
Therefore, there is an urgent need to improve the performance of conformal holographic AIS antennas to make them more viable for commercial applications with improved efficiency, simplicity and compactness.